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If I fill up (NOT overfill) the tank, there's fuel dripping from the right side, just in front of the rear wheel. The dripping would stop after 10 minutes. Also I experienced this: with tank nearly full (I'd say about 90-95%), I parked my car in a sloping surface, so that the car tilts to the right. When I got out, fuel drips more than before, it's like a rain. Workshop guy changed the fuel pump and the seal but the problem persisted. He said he must detach the tank to see what's wrong and it would take 2-3 days and it's rather costly. So before I do that, maybe you guys could diagnose what's wrong? Also, could this cause poor fuel economy like I'm experiencing now? Note: both troubles already existed when I bought the car, so I can't decide if one thing leads to the other.

Well if your fuel is dripping out onto the road instead of being used by the engine then yes it will give you bad fuel economy.

There is an inspection hole where you can see the top of the tank under the rear seat (lift up seat, remove screws holding metal plat on and have a look). Also try look under the car and see where its coming from, go to realoem.com ...http://www.realoem.com/bmw/select.do?kind=P&arch=1
Eneter your car details and find the fuel tank diagram, might give you more idea of where it may be coming from

Just one caveat
a empty tank is the most dangerous !
It is the fumes that ignite and go Kapooof.

I had a buddy of mine remove his spare tank, as he wanted to make it smaller.
He dumped all the fuel and washed it out with water & dishwash twice.
Then he let it stand overnight with the opening facing straight down.
Next morning he took the grinder too it and woke up that afternoon in hospital

I think if I ever work on a fuel tank Im going to intentionally ignite it from a safe distance first. Heard way too many stories of people washing them out letting them sit for ages etc only to be greeted with a big bang.

I purge fuel tanks before I do work on them in any way that could cause spark
Fancy way is to pump the tank full of a inert gas, such as CO2 and then begin.
Then there is the Africa way, where you pump the exhaust fumes from another car (preferably Diesel) into the fuel vessel before welding, cutting grinding...
Over here we cannot sue somebody for our own stupidity

Just got my car checked by a mechanic and he said there's no problem with the tank and hoses. Then we went to a gas station fill the car up. When the fuel starts dripping he removes the backseat and we see the fuel floods out from there (below the right seat, where there are pump seal and pipes). I'm not sure where it comes from but surely it's not a tank problem, right? Any idea where?

I've changed the seal like twice and it still leaks. Apparently there are two sources of leak. The pump (below the cover) and somewhere below the left seat, which can't be seen except if the whole tank getsdetached